Letter pubs.acs.org/NanoLett
Three-Dimensional in Situ Photocurrent Mapping for Nanowire Photovoltaics Patrick Parkinson,*,† Yu-Heng Lee, Lan Fu, Steffen Breuer, Hark Hoe Tan, and Chennupati Jagadish* Department of Electronic Materials Engineering, Research School of Physics and Engineering, The Australian National University, Canberra, ACT, 0200, Australia S Supporting Information *
ABSTRACT: Devices based upon semiconductor nanowires provide many well-known advantages for next-generation photovoltaics, however, limited experimental techniques exist to determine essential electrical parameters within these devices. We present a novel application of a technique based upon two-photon induced photocurrent that provides a submicrometer resolution, three-dimensional reconstruction of photovoltaic parameters. This tool is used to characterize two GaAs nanowire-based devices, revealing the detail of current generation and collection, providing a path toward achieving the promise of nanowire-based photovoltaic devices. KEYWORDS: GaAs, nanowire, photovoltaic, photocurrent mapping, two-photon optical beam induced current (TOBIC)
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(rather than one for planar photovoltaics), and a full experimental characterization of the spatial variation in photovoltaic behavior is lacking. Tools such as electron-beam induced current (EBIC) provide high spatial resolution and important information about fundamental physical processes in model (ex situ) systems;12,13 however, they lack the depth resolution required for next-generation devices such as nanowire-based solar cells. An ideal technique to provide this is two-photon optical-beaminduced current (TOBIC).14 Here, a nonlinear two-photon absorption process allows selective injection of photocurrent with three-dimensional resolution, which has been used in silicon-based electronics providing sub-100 nm spatial resolution under highly optimized optical conditions.15,16 In this Letter, we use the TOBIC technique to selectively inject photocarriers into a nanowire photovoltaic device with submicrometer-scale resolution and three-dimensional selectivity. Furthermore, by performing photocurrent−voltage measurements, quantitative photovoltaic parameters can be extracted. This technique provides important information for design-led optimization of nanowire photovoltaic devices. The two nanowire photovoltaic devices analyzed here were prepared using gold-assisted growth (via the vapor−liquid solid (VLS) mechanism) in a metal−organic chemical-vapor deposition reactor.17 While approaches based upon selectivearea grown structures allow for greater control over the positioning of nanowires,8 VLS growth is a low-cost, mask-free, bottom-up and high-quality approach with random nanowire
hotovoltaic devices based upon nanowires have shown particular promise for decreasing material costs, increasing light absorption through optical scattering,1 exploiting multiple junction cells less restrained by lattice matching conditions,2 and increasing the volume of p−n junction through effective structuring of the surface.3−5 Of particular interest are photovoltaics based around GaAs nanowires, owing to their good absorption overlap with the solar spectrum, high carrier mobility6 and, with appropriate passivation, long carrier lifetime.7 Recently, through the use of an in situ engineered conformal contact layer, power conversion efficiencies of over 4% have been shown,8 revealing that via a thorough physical understanding of the optical and electrical performance of individual devices, appropriate engineering steps can make improvements toward the eventual promise of nanowire solar cells. From a theoretical standpoint, techniques such as finitedifference time-domain (FDTD) simulation give a deeper understanding of optical effects within model structured materials, providing in-depth three-dimensional insights for physics-led design of devices.9,10 Alternatively, finite element analysis can aid geometrical and device design,11 while systematic device-based simulations provide theoretically optimized geometrical and material parameters.2 From the experimental side, scanning photocurrent mapping can provide spatially resolved experimental information about chargegeneration efficiency on the nanoscale, with two-dimensional resolution.12,26 Using these approaches, we are able to calculate the predicted carrier density and dynamics within a wire, and the experimental photocurrent from plane-view excitation. However, the full promise of nanowire photovoltaics relies on the varied, complex, and buried junction that such devices provide. The depletion region varies in three-dimensions © 2013 American Chemical Society
Received: November 12, 2012 Revised: January 14, 2013 Published: March 6, 2013 1405
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placement; for nanowires of small diameter, there is evidence that random positioning (or aperiodicity) is advantageous for broadband antireflection behavior.18 Nominally undoped gallium arsenide (GaAs) nanowires (NWs) were grown on pdoped (111)B GaAs substrate using 50 nm gold colloids as a catalyst, following an optimized two-temperature procedure previously reported.19 The wires were grown to be 6−8 μm in length. Device 1 and Device 2 varied only the subsequent epitaxy steps; while the former was coated with an n-doped AlxGa1−xAs shell grown with x = 26% (in the vapor) at a temperature of 650 °C, Device 2 was coated with an n-doped AlxGa1−xAs shell layer grown with parameters optimized for surface passivation (x = 50% in the vapor, at 750 °C), following the high-quality procedure described by Jiang et al.7 The dopant was provided using a silane source and was expected to lead to carrier densities of ND = 3 × 1017 cm−3, based upon previous thin-film characterization. Both device growths were followed by a heavily n-doped GaAs capping layer (ND = 1 × 1018 cm−3) for contacting and prevention of oxidation of the AlGaAs layer. It must be noted that during the growth of the two shell layers, both radial deposition and (to a lesser extent) planar deposition onto the substrate between the nanowires is expected. Using scanning electron microscopy (SEM) and transmission electron microscopy (TEM, data not shown), it was determined that the nanowires were minimally tapered, 6− 8 μm in length and of a total diameter of approximately 70 nm, with homogeneous sidewalls evidencing good crystal quality. Following the growth, the devices were fabricated by planarisation using Benzocyclobutene (BCB) resin. The resin was applied using spin-coating and thermal curing, followed by inductively coupled-plasma reactive-ion-etching to reveal the nanowire tips. Finally, the top surfaces were contacted using 200 nm thick, sputter-deposited ITO (of sheet resistance 100 Ω/□ and 85% transmission) and a gold contact, while the bottom surfaces were contacted using Ti/Au deposition (10 nm/200 nm) using electron-beam evaporation. A schematic of the photovoltaic structure is depicted in Figure 1, along with typical scanning electron micrographs of a complete device.
Figure 2. Photocurrent maps for above-gap photoexcitation. (Top) Large area, (middle) intermediate area, and (bottom) small-area maps are shown for each device. The lower efficiency Device 1 is shown on the left, while the higher efficiency Device 2 is on the right. All photocurrent scale bars are in units of nA/μW.
currents of 0.018 and 9.79 mA cm−2 were calculated for the two devices with a corresponding efficiency of 0.04 and 3.56%, respectively; however, one should note that the area used was the planar dimension of the sample, including the areas between the nanowires, but excluding areas where the nanowires remain predominantly covered with BCB and thus uncontacted. The experiment consists of a Yb:YAG laser, outputing ∼300 fs laser pulses at 1044 nm (1.188 eV) with a 20.8 MHz repetition rate. The laser is optionally frequency doubled to 522 nm (2.375 eV) using an LBO crystal, and the pulse is directed through a computer controlled neutral density filter, an optical chopper (at 300−400 Hz) and a 0.75NA 100× long-working distance objective to a focal spot of around 1 μm on the sample. The laser power at the sample may be varied between a maximum of 1 nJ/pulse, down to 10 pJ/pulse for either 522 or 1044 nm excitation. The detected photocurrent is amplified using a current preamplifier and measured using a lock-in amplifier, with the typical noise level being below 0.1 nA. The sample was positioned below the laser using piezo motors (xy) or a geared stepper (z) for each axis, providing 0.2 and 0.3 μm positioning accuracy, respectively. The TOBIC process relies on two-photon absorption to generate carriers specifically at the focal voxel and not above or below this within the focus cone (as would occur for above-gap excitation). To confirm our identification of 1044 nm as “below-gap” (two-photon) absorption and 522 nm as “abovegap” (one-photon) absorption, a laser power dependent photocurrent measurement was carried out (results given in the Supporting Information). Neither curves show ideal behavior of the form I ∝ Pα (where α = 1 for one-photon and 2 for two-photon absorption processes, respectively), however, this can be attributed to device-related issues due to
Figure 1. A schematic and plane-view/45°-view scanning electron micrograph of a nanowire photovoltaic sample. The random distribution and density of nanowires can be observed. The scale bars for the micrographs are 10 μm.
The photovoltaic devices were initially characterized using current−density voltage measurements in the dark, and under simulated AM1.5G illumination at room temperature using an Oriel solar simulator (see Supporting Information for data). Open circuit voltages of 0.26 and 0.7 V were exhibited for Device 1 and Device 2, respectively. By considering only the high-current region of the devices seen in Figure 2, short-circuit 1406
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Figure 3. A series of slices through a three-dimensional reconstruction of the photocurrent generated in (upper) Device 1 and (lower) Device 2; the same color scale is used along each row, with the scale bar shown at the right in nA. The left-most figures are slices through the xy plane. The figures on the right are slices through the yz and xz planes indicated by gray lines on the left-hand figures, revealing information in the third dimension.
hand, in Device 2 the majority of the photocurrent is generated at the substrate, as clearly indicated by the large photocurrent signal at the z = 0 position. However, while the map in Figure 2 indicates reduced current at the nanowire positions, the TOBIC scan reveals that for Device 2 the nanowire itself does contribute current to the cell performance, albeit less than the substrate junction. The physical origin of the strong contribution of the substrate to the photocurrent is discussed later. The true strength of the TOBIC is in disentangling the photocurrent generated at these different positions; while direct comparison between the samples can be complicated by slight changes in system alignment and laser power, it is clear that the nanowires in Device 2 produce between 20× and 40× as much photocurrent as the nanowire in Device 1. This is in striking agreement with the 20× to 30× enhancement in minority carrier lifetime observed for similarly AlGaAs passivated samples in ref 7. Such detail is not available using other characterization techniques. It is also noted that for the particular nanowire studied on Device 1, significant photocurrent can be observed at the tip of the nanowire; this effect arises occasionally for both samples, but does not occur in all measured regions. The origin of this response is attributed to enhanced two-photon absorption occurring near to the gold nanoparticle at the tip of the nanowire. Such an enhancement is expected due to the strongly geometry-dependent increase in local electric field strength near to gold nanoparticles (see, for instance ref 20). A full-scale FDTD model is often required to understand plane-wave excitation of nanoscale objects, due to the failure of conventional ray optics arising from geometrical effects on subwavelength scales.10,21 To an extent, the TOBIC technique mitigates against such effects by using excitation with a voxel of small spatial extent. In addition, care was taken to measure areas with low nanowire density to avoid optical interactions arising from ensemble interactions. More quantatitive information can be extracted by use of a high-resolution single-axis (z-axis) scan, as shown in Figure 4. Two z-axis scans are shown for Device 1, one for a nanowire position and one for a substrate position. First, a fairly significant background signal can be observed away from the nanowire/substrate interface (at z = 0). This is attributed to multiphoton absorption in the substrate and ITO, along with residual 522 nm illumination (arising from optical scattering). By subtracting the signal at the substrate from that at the
the relatively high local current injection under the experimental illumination conditions. It remains clear that above-gap excitation displays a sublinear characteristic (α = 0.8), while below-gap exitation shows superlinear behavior (α = 1.3), as expected for a two-photon absorption dominated process. It should be noted that throughout the experiments described herein, the excitation power used produced photocurrent equivalent to above-gap excitation with a fluence of 0.5−100 μW/μm2; this is significantly greater than AM1.5 illumination and is required to provide a measurable photocurrent. However, such excitation intensities are fairly common for both nanowire photovoltaic photocurrent mapping26 and single nanowire optoelectronic characterization.6,7,25 It is hoped that future optimization to the technique may be able to produce data under near-solar excitation conditions. Initially large and small area photocurrent maps of both devices were produced using above-gap excitation with an average laser power of 35 ± 1 (1.1 ± 0.2) μW for Device 1 (Device 2), respectively. Figure 2 shows these results with a focus upon a typical region for each device. A fundamental difference in the spatial arrangement of charge generation can be seen between these two regions; while Device 1 shows that photocurrent is predominantly generated within the nanowires, Device 2 shows that the substrate between the nanowire dominates the photocurrent contribution. Both regions have good contacting and planarisation with no electrical shorting observed using SEM micrographs (data not shown). As neither the physical origin of the improved charge generation efficiency between the wires nor the absolute photocurrent contribution of the nanowires themselves are immediately clear from these images, they represent an ideal situation for the application of the TOBIC method. Three-Dimensional Photocurrent Mapping. For each device, a three-dimensional photocurrent map was made using a voxel size of (300 nm)3 and a 1044 nm laser power of 940 ± 10 μW. Figure 3 shows four two-dimensional slices through each of these maps. In each, the photocurrent (in nanoamps) is revealed for excitation in one of the three planes, xy parallel with the substrate, and yz/xz, perpendicular to the substrate and in the plane of the nanowire. The advantages of the TOBIC technique are striking; it is immediately clear that the entire current generated in Device 1 comes from the nanowires with almost no current generated at the substrate. On the other 1407
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photovoltaic performance of Device 2 over Device 1, a laserpower dependent IV experiment was carried out, as shown in Figure 5. While Device 1 shows a poor Voc, fill factor, and peak
Figure 4. (Top) z-axis dependent photocurrent for two points in Device 1 as shown in the inset; at a nanowire (black circles) and at the substrate (red circles). (Below) The difference signal is shown. Vertical lines represent the substrate (at z = 0 μm) and the tip of the nanowires (at z = 7 μm). The red line is a 3.2 μm monoexponential decay fit to the data, while the black line is a half-Gaussian fit to the nanowiresubstrate interface with a half width at half-maximum of 0.76 μm.
Figure 5. The laser power dependence of the photocurrent−voltage response for both (upper) Device 1 and (lower) Device 2 is shown for 1044 nm excitation. At low powers (left), it can be seen that a fairly typical IV response is shown, while at high powers (right) an atypical response, attributed to contribution from AlGaAs is observed.
nanowire position, a differential photocurrent can be resolved. Here, photocurrents of up to 8 nA are generated within the single nanowire with a spatially varying response. Within the length of the nanowire, the photocurrent can be seen to drop approximately exponentially over the length of the nanowire (indicated with a red line) with an exponential length scale of 3.2 μm. The change in signal at the substrate-nanowire interface occurs over a half-width at half-maximum length scale of ∼0.76 μm, revealing the exceptional spatial resolution of the TOBIC technique. However, further interpretation is challenging and requires caution due to both the finite size of the excitation voxel and the effect of the refractive index variation within the sample. While in theory voxel sizes of significantly less than the wavelength are achievable through use of high numerical aperture lenses, solid-immersion lenses, or beam aperture engineering,15 no such action is taken here. A comparison of two-dimensional photocurrent maps obtained under both above- and below-gap conditions is presented in the Supporting Information, revealing a negligible change in spatial resolution, around 0.5 μm half width at half-maximum in both cases. In future, the aforementioned resolution enhancing techniques may be used to achieve the full promise of strong subwavelength imaging. It is known that the minority carrier diffusion length in both n-type and p-type GaAs nanowires is of the order of 100 nm (measured for axial p−n junctions13); measurement of such dimensions are beyond the scope of the experiment as presently designed. Typically, a reduction in carrier collection efficiency may be expected for injection toward the top of the nanowires, due to the longer exposure of holes to recombination at bulk or surface defects as they travel to the collection substrate;6 more efficient charge collection is to be expected closer to the base of the nanowires. Photocurrent−Voltage Measurements. While a highresolution, three-dimensional in situ photocurrent map is of inherent use for visualizing and characterizing the active volume for each nanowire within the device, further quantitative results can be obtained from photocurrent−voltage (IV) measurements. Using this technique, carriers are selectively injected at a specific point within the device, and an IV experiment is performed. To further investigate the origin of the improved
power output under all illumination intensities, a clear change with excitation intensity is observed for Device 2. At low excitation power, a rather typical IV curve can be seen with Voc ≈ 0.8 V and a fill factor of around 36%, typical for GaAs nanowire-based photovoltaics8,22 (a full analysis is presented in the Supporting Information). The value of Voc at all powers is slightly larger than that measured under AM1.5G conditions; this is to be expected due to excitation directly at the nanowire position with carrier energy significantly above the band gap with high power. Under higher injection intensity, a significant quantatitive change in response is observed. Here, a second material response appears, leading to a large open-circuit voltage Voc = 1.3 V; this is significantly larger than the highest obtained for GaAs cells23 and is therefore likely to arise from the AlGaAs passivation layer. While these devices were not designed to use the AlGaAs passivation layer as an absorber, it can be seen that under the high injection conditions present this layer may contribute to the photovoltaic performance. To our knowledge, while tandem axial nanowire photovoltaics have been studied,24 such performance has not been observed to date from radial-heterostructure GaAs nanowire-based photovoltaics. While photovoltaics based upon nanowires are known to have beneficial properties arising from the nanoscale structuring, the challenges and possibilities of such devices are yet to be realized. As idealized models give way to full electromagneticand device-based models,2,11 the impact of real-world features such as substrate p−n junctions and nanowire inhomogeneity are becoming increasingly important. The TOBIC-based technique presented in this Letter provides a deep experimental insight into both spatial current generation (through threedimensional mapping) and the mechanisms behind generation and recombination (through photocurrent−voltage response). In the two studied devices, a drastically different photoresponse was observed. While Device 2 appears to provide higher photovoltaic efficiency, the dominant contribution of the substrate suggests that the nanowires play a relatively small but significant role in photocurrent generation. Conversely, 1408
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ACKNOWLEDGMENTS The authors thank Jens Ö lmedal for SEM images, and Zhe Li for solar simulator photovoltaic measurements. The authors acknowledge the financial support of the Australian Research Council. Facilities used in this work are supported by the Australian National Fabrication Facility.
Device 1 has a nanowire-dominated response, along with photocurrent from throughout the full length of the nanowire as expected, but a lower overall efficiency. Indicated in Figure 1 are two typical paths for photogenerated carriers; where the electron−hole pair is generated in the nanowire, the holes are required to travel through the nominally undoped (background lightly n-type) nanowire core, while the electrons can drift to the n-doped shell of the nanowires. Alternatively, in the case where a parasitic n-doped epitaxial layer is deposited between the nanowires during shell growth, electron-hole pairs generated at this interface may separate and be collected through their respective majority-carrier material. If no such epitaxial layer is deposited, or such a layer is of very low quality, such collection is not possible. These two situations are likely to describe Device 2 and Device 1, respectively. It is known that at 750 °C, a higher quality AlGaAs layer is likely to be deposited. In addition, at higher temperatures parasitic growth is also more likely to occur. In combination with the higher opencircuit voltage of Device 2 with respect to Device 1, and the power-dependent photocurrent−voltage measurements in Figure 5, it seems that the primary effect of the improved AlGaAs layer (and shell) are 3-fold: • To act as a surface passivation layer, improving the interface quality of the nanowires as evidenced by both the increased photocurrent under identical conditions and increased Voc of Device 2 with respect to Device 1 • To provide complete surface coverage, increasing the photocurrent contribution of the planar interface between the nanowires • To act as a secondary absorber, provided a tandem-cell like response under high excitation power conditions In conclusion, a two-photon induced photocurrent and photocurrent−voltage response has been presented for two GaAs nanowire photovoltaic devices. This technique provides three-dimensional photocurrent response maps, capable of both validating theoretical models and experimentally revealing the performance of such devices. When coupled with photocurrent−voltage responses, a three-dimensional model of charge generation, separation, recombination and collection may be obtained. This technique provides a unique opportunity to investigate the working mechanism of the complex, irregular, nanostructured photovoltaics that present such high promise as next-generation photovoltaic devices.
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REFERENCES
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ASSOCIATED CONTENT
* Supporting Information S
The AM1.5G photovoltaic response of both devices, a laserpower photocurrent dependence, and a calculation of the spatial resolution of the TOBIC technique are provided. This material is available free of charge via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: (P.P.)
[email protected]; (C.J.) cxj109@ physics.anu.edu.au. Present Address †
University of Oxford, Department of Physics, Clarendon Laboratory, Parks Road, Oxford OX1 3PU, United Kingdom. Notes
The authors declare no competing financial interest. 1409
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